The computer industry, in general, and the workstation and personal computer (PC) markets in particular, have been moving rapidly towards the use of interfaces that are standards-based. A standards-based interface is one which has been adopted by the industry as a common denominator, thereby assuring that any device exhibiting that interface will be able to communicate with another device having the same interface. In the area of peripherals, and especially disk drives, the issue of plug-to-plug hardware compatibility is extremely important, as such peripherals are often purchased as stand-alone devices for use with other data processing equipment.
One widely employed standard interface is the Small Computer System Interface (SCSI) which is implemented by a parallel, multimaster, I/O bus between a computer and a peripheral device. Among other features, the SCSI interface enables a disk controller to hide many of the details of disk operation from a host computer. The computer does not have to manage the details of where the file is physically located and does not have to separate header and trailer information from the data.
The SCSI interface was designed and adopted at a time when the norm was a single PC connected to a single disk drive. Today, disk drives are being integrated into multi-user systems. In such an environment, known problems with the SCSI interface that were present, but ignored, in the environment of single PC - single disk drive systems, cannot be ignored. Thus, in a multi-user system, the user expects, in the event of a power failure, that stored data will be protected and that the system will be able to easily recover. This protection feature is referred to as sector atomicity. Sector atomicity requires that a disk drive will complete a write of a physical sector on a disk under all conditions, even in the event of loss of supply voltage to the drive. Thus, sector atomicity assures that data being written, under all conditions, is actually written into non-volatile memory. Thus, upon a re-start, it is clearly known where to recommence operations and no data is lost.
The SCSI interface does not address this issue. In a single PC/disk drive system, if a power failure occurs, data is lost--but that is the user's problem, to be solved by a re-booting of the system and re-entry by the user of the lost data.
The SCSI interface standard does not provide for a power fail imminent signal. Therefore, alternative techniques have been suggested to assure sector atomicity. It is known that certain disk drives, in order to complete the write of a sector, require at least 330 microseconds of valid supply voltage. Thus, given sufficient warning of a power fail, such a disk drive can complete a sector write (and prevent new writes from starting)--thereby enabling ready recovery once power is restored. It has been suggested that power supply monitoring circuitry be included in the disk drive. It has been found, however, that there is a great variation in voltage decay times in the event of a power fail. Thus, there can be no guarantee that such monitoring circuitry can provide the minimal pre-notice of the power failure and assure a valid supply voltage for the required duration.
Disk drive power supplies do provide, as a feature, a signal output that predicts an imminent power failure. That output generally provides greater than 400 microseconds of warning of a power failure. Such power supplies accomplish this function by monitoring supply levels and have capacitor sizes that store sufficient charge to temporarily support the output level, even in the event of A/C supply failure. Notwithstanding the availability of an imminent power fail signal, SCSI and other standard interfaces do not provide the capacity to handle such.
Accordingly, it is an object of this invention to provide a disk drive having an industry standard interface with the capability to respond to a power fail signal and assure sector atomicity.
It is another object of this invention to provide a disk drive that invariably insures sector atomicity upon a power fail without requiring external alteration to the disk drive package.